MR imaging of internal body tissues may be used for numerous medical procedures, including diagnosis and surgery. In general terms, MR imaging starts by placing a subject in a relatively uniform, static magnetic field. The static magnetic field causes hydrogen nuclei spins to align and precess about the general direction of the magnetic field. Radio frequency (RF) magnetic field pulses are then superimposed on the static magnetic field to cause some of the aligned spins to alternate between a temporary high-energy non-aligned state and the aligned state, thereby inducing an RF response signal, called the MR echo or MR response signal. It is known that different tissues in the subject produce different MR response signals, and this property can be used to create contrast in an MR image. One or more RF receivers detect the duration and strength of the MR response signals, and such data are then processed to generate tomographic or three-dimensional images.
MR imaging can also be used effectively during a medical procedure to assist in locating and guiding medical instruments. For example, a medical procedure can be performed on a patient using medical instruments while the patient is in an MRI machine. The medical instruments may be for insertion into a patient or they may be used externally but still have a therapeutic or diagnostic effect. For instance, the medical instrument can be an ultrasonic device, which is disposed outside a patient's body and focuses ultrasonic energy to ablate or necrose tissue or other material on or within the patient's body. The MRI machine preferably produces images at a high rate so that the location of the instrument (or the focus of its effects) relative to the patient may be monitored in real-time (or substantially in real-time). The MRI machine can be used for both imaging the targeted body tissue and locating the instrument, such that the tissue image and the overlaid instrument image can help track an absolute location of the instrument as well as its location relative to the patient's body tissue.
MR imaging can further provide a non-invasive means of quantitatively monitoring in vivo temperatures. This is particularly useful in the above-mentioned MR-guided focused ultrasound (MRgFUS) treatment or other MR-guided thermal therapy where temperature of a treatment area should be continuously monitored in order to assess the progress of treatment and correct for local differences in heat conduction and energy absorption. The monitoring (e.g., measurement and/or mapping) of temperature with MR imaging is generally referred to as MR thermometry or MR thermal imaging.
Among the various methods available for MR thermometry, the proton-resonance frequency (PRF) shift method is often preferred due to its excellent linearity with respect to temperature change, near-independence from tissue type, and high sensitivity. The PRF shift method is based on the phenomenon that the MR resonance frequency of protons in water molecules changes linearly with temperature. Since the frequency change is small, only −0.01 ppm/° C. for bulk water and approximately −0.0096 to −0.013 ppm/° C. in tissue, the phase shift is typically detected with a phase-sensitive imaging method in which the imaging is performed twice: first to acquire a baseline phase image prior to a temperature change and then to acquire a second image after the temperature change, thereby capturing a small phase change that is proportional to the change in temperature.
A phase image, for example, may be computed from an MR image, and a temperature-difference map relative to the baseline image may be obtained by (i) determining, on a pixel-by-pixel basis, phase differences between the phase image corresponding to the baseline and the phase image corresponding to a subsequently obtained MR image, and (ii) converting the phase differences into temperature differences based on the PRF temperature dependence while taking into account imaging parameters such as the strength of the static magnetic field and echo time (TE). It should be appreciated that, although a subtraction step may be involved, the determination of the phase differences involves more than a simple subtraction of scalars.
Unfortunately, changes in phase images do not arise uniquely from temperature changes. Various non-temperature-related factors, such as changes in a local magnetic field due to nearby moving objects, magnetic susceptibility changes in a patient's body due to breathing or other movements, and magnet or shim drifts can all lead to confounding phase shifts that may render a phase-sensitive temperature measurement invalid. For example, during some MR-guided thermal treatment procedures, one or more treatment devices need to be re-positioned and/or re-oriented in or near the MR imaging area. Since the treatment devices typically include metal components, their movements could perturb local magnetic fields and thereby significantly change the phase background. Non-metal objects and their movements may also perturb local magnetic fields. The patient's breathing or turning motions, for example, could have similar effects. In fact, the changes in magnetic field associated with patient motion and/or nearby objects can be severe enough to render temperature measurements made using the above-mentioned phase-sensitive approach useless.
Further measurement artifacts may arise from “phase wrapping” when significant temperature changes are monitored over a long time period, such as during a lengthy treatment procedure. In existing PRF-based temperature measurements, each phase image is compared directly with an initial baseline reference to determine the amount of phase shift. However, if the accumulated phase shift has exceeded π in any part of a recently acquired phase image—a condition referred to as “phase wrapping”—the true magnitude of phase shift will not be captured by simply subtracting the baseline reference image from the recently acquired phase image.
In view of the foregoing, it may be understood that there are significant problems and shortcomings associated with current PRF techniques.